Asphalt may be the most recycled material in the United States. In fact, tens of millions of tons of asphalt pavement removed each year during highway widening and resurfacing projects is reused as pavement. Such recycling efforts conserve natural resources, decrease construction time, minimize the impact of asphalt plant operations on the environment, and reduce reliance on landfills. Further, research shows that the structural performance of mixtures integrating reclaimed asphalt pavement (“RAP”) is equal to, and in some instances better than, virgin asphalt pavement.
Over time, various methods for in-place recycling of asphalt pavement have evolved, including but not limited to hot in-place recycling, cold in-place recycling, and full-depth recycling. These recycling processes generally involve mechanically breaking up a paved surface, applying fresh asphalt or asphalt rejuvenation materials to the pieces, depositing the resulting mixture over a road surface, and compacting the mixture to restore a smooth paved surface. In some cases, broken asphalt may be removed from a road surface, treated off location, and then returned and compacted.
Due to the rigid and abrasive nature of cold asphalt, the hardness of which may approach concrete, heat may be applied to a paved surface prior to milling, grinding, or otherwise working the surface. The heat may be used to soften the asphalt and reduce the wear and tear on asphalt working equipment, as well as reduce the power needed to operate such equipment. Such heat may be applied using direct-flame, radiant, or other suitable types of heaters, which generally rely on the principle of conduction for heat to penetrate the paved surface. Such reliance on conduction generally requires application of heat for long periods of time in order to heat the pavement to sufficient depths. This prolonged exposure generally produces a significant downward temperature gradient in the pavement. Furthermore, the amount of heat that may be applied is severely limited due to the possibility of burning, igniting, or damaging the asphalt.
In order to address some of these problems with conventional heating, some have experimented with microwaves to heat asphalt and other pavement constituents. Rather than relying on conduction, the microwaves penetrate the pavement to excite water or other excitable constituents substantially evenly through the pavement. This enables faster heating of the pavement since constituents at various depths are excited together. Nevertheless, asphalt materials are generally not very responsive to heating by microwave energy. Aggregate materials are typically more responsive to microwave energy and, once heated, may heat the surrounding asphalt materials by conduction.
Nevertheless, like conventional heating methods, microwave energy may also produce a temperature gradient in the paved surface, although the gradient may be reversed and less severe than heating by conduction. That is, microwave energy tends to heat deeper regions of the paved surface more effectively than the surface. This inverted gradient may be due in part to moisture evaporation at the surface in addition to the more rapid cooling that occurs at the surface. This inverted gradient may occur in various types of old and weathered pavement, which may develop a hard dehydrated crust over time due to the evaporation of water or other volatile constituents in the asphalt binder.
To address some or all of the above-stated problems, improved apparatus and methods are needed for heating paved surfaces using microwave energy. More particularly, apparatus and methods are needed to improve the efficiency and uniformity of heat applied to paved surfaces using microwave energy. Further needed are apparatus and methods for restoring moisture to dry and dehydrated pavement to make the pavement more conducive to microwave heating. Further needed are apparatus and methods to remedy the inverted gradient that may occur when using microwaves to heat paved surfaces.